The use of dry gas seals in process gas centrifugal compressors has experienced a dramatic increase over the past couple of decades. Most centrifugal gas compressors manufactured and sold today have dry gas seals.
As the shaft must have clearance within the bearings to allow low-friction operation, some type of shaft sealing must be utilized in order to prevent higher pressure process gas from escaping the compressor housing freely. This escape may represent a contamination of the work environment or the global environment, or simply represent an inefficiency and loss of valuable compressed process gas.
Typical multi-stage compressors require at least two seals, one at opposing ends of the shaft. Dry gas seals are face seals that consist of a rotating ring and a stationary ring. During normal operation, fluid dynamic forces cause a gap between these rings. Some type of sealing gas is then injected into this gap and provide a seal between the external atmosphere (or sometimes a flare system) and the compressor internal process gas. Often there is an internal labyrinth arrangement that separates the process gas from the seal gas.
An expander-booster machine may use gas bearings to prevent oil ingress into the process stream and an temperature migration between the expander and the booster. Process air may also leak from the higher pressure of the booster side to lower pressure of the seal gas side. A significant portion of these leak and seal gases can ordinarily be recovered at medium pressure and re-injected at BAC suction.
The above comments regarding dry gas seals, and their common applications, are not intended to limit the scope of the claimed invention. The comments are made by way of explanation only. For example, the hereinafter described and illustrated preferred embodiments of dry gas seals could be used in other applications and/or in conjunction with gases other than those mentioned above.
In a process scheme at high pressure with a single machine, the main air source pressure is too high to allow the expander-booster air losses to be recovered. These losses are thus vented to atmosphere. As a result the corresponding air flowrate is compressed by the MAC and not used further in the process. The corresponding power loss can reach 1% of the total compression power of the plant, depending of the size of the expander booster. There is a need in the industry for a method that can allow these power losses to be significantly reduced.